In some locations, it may be necessary to build drilling rigs very close to each other due to legal or natural land restrictions. The wells may then be directionally drilled underground to cover a vast region of land. In these cases, it is important to locate the presence of other wells to prevent hazards such as blow outs. However, if the well that is being drilled along a path that intersects another well (for example, a horizontal well in the presence of vertical wells), it may be difficult to ascertain the position of the second well with traditional methods.
Therefore, determining the position and direction of a conductive pipe (metallic casing, for example) accurately and efficiently is required in a variety of downhole applications. For example, one application is in the case of a blown out well in which the target well must be intersected very precisely by a relief well in order to stop the blowout. Other applications include well avoidance in which, as a well is being drilled, another well is detected and avoided.
A number of conventional approaches have attempted to provide solutions to this problem. In one method, coil antennas are used as transmitters, the amplitude of the fields induced on the target casing is measured, and a determination of the position and distance of the target is made based on these measurements. However, since the amplitude of the field is strongly dependent on the properties of the casing and the formation, the accuracy of this method may remain low. In another approach, an electrode type source is used to induce current on the target casing. Gradient of the magnetic field radiated by the target casing in addition to the magnetic field itself is measured in this approach. By using a relationship between the magnetic field and its gradient, the range to the target casing may be determined and be used for avoidance or intersection. However, this method can be insensitive if a true T-intersection occurs due to a cancellation of the fields at the receiver location.
Accordingly, there is a need in the art for improved downhole ranging techniques useful in T-intersection scenarios.